A variety of mobility aids have been developed to provide the visually impaired with a means of travelling independently, confidently and quickly. The most commonly used mobility aid is the white cane. The white cane is an inexpensive and effective device for providing the user with information on some of the environment within the range of the cane. Some of the disadvantages of the white cane are that obstacles located above ground level are not easily detectable and obstacles located outside of the range of the cane are undetectable.
Recently developed mobility aids have taken advantage of advances in microelectronics to address the drawbacks associated with the white cane. One such mobility aid, known as the "laser cane", consists of a white cane incorporating three lasers, each directed in a different direction in the vertical plane. The three laser beams are referred to as the upper beam, the straight ahead beam and the lower beam. When an object is located at a specific distance in the beam's path, some of the reflected beam is received by an optical receiver positioned at a fixed distance from the laser on the cane. When a reflected laser beam is detected by the receiver, an audible tone is produced. There are separate receivers for the upper and straight ahead beams and each causes a different audible tone to be produced when a beam is detected. The lower beam operates differently from the upper and straight ahead beams since the lower beam is continually reflected by the ground to its own receiving unit on the cane. Thus when a drop off is encountered the receiving unit will not detect a reflected beam and it will cause an audible tone to be produced, distinct from the tones associated with the upper and straight ahead beams. A distinctly different tone is produced for each beam and by interpreting the tones the user can determine the position of the object. In addition, the user has the option of also utilizing a vibrating unit on the cane which provides tactile information to the user when an object is detected by the straight-ahead beam. One drawback with this device is that its use of a fixed triangulation method only permits the detection of objects at specific distances, one for each laser, from the user.
In yet another prior art device, the "BRYTECH* Sensory 6", ultrasonic waves are used to detect the presence of an object in the vicinity of the user. The device emits ultrasonic waves and by detecting the reflected waves determines the proximity of the object to the user. Periodic tone pulses, about two per second, are produced, the frequency of each pulse representing the average distance from the user of the area from which the waves are reflected. A near scene will cause a high-pitched tone and a distant scene will cause a low-pitched tone. These tones are provided to the user in earphones. A limitation of this device is that the ultrasonic waves emitted by the device have a wide pattern, and consequently it is difficult for the user to determine
* BRYTECH - trade mark the position of an object in the detection pattern. A further limitation of the device is that confusion can arise due to the inability of the device to distinguish between objects at different distances which are in the field of view at the same time.
In yet another prior art device, the "MOWAT* Sensor," an ultrasonic beam is emitted having a detection pattern approximating the size of a typical person. If an object is present within the pattern, the reflected wave is received by the device and the hand held sensor is caused to vibrate. The closer the object to the user, the higher the vibration rate of the sensor. If two or more objects are located within the beam, the device will respond in a manner dependent on the relative sizes of the objects. The sensor has a short and a long range setting. On the short range setting, the device will only respond to objects within a short distance of the device, whereas in the long range setting the device will respond to objects at a greater distance. One of the drawbacks of this device is that the large size of the detection pattern does not enable the user to locate objects accurately. Another drawback of the device is that it does not readily enable the user to interpret the panorama of the surrounding environment or objects located therein.